Color correction



D'NJ/TY 9 DENSITY" EXPOSURE EXP05JRE 2/ /8 PRINT/N6 PR/IYTl/YG I 20 Jan. 1942- J. A. c. YULE 2,268,791

COLOR CORRECTION Filed April :50, 1940 2 Sheets-Sheet 1 F/e. F/s. 2

EXPOSURE THROUGH EXPOSURE THROUGH F/LTER x /7 FILTER Y MASK/N6 4 v oflvs/ry-cwe NEUTRAL l2/N6 22 EXPOSURE DENSITY vs EXPUSI/Rf 7 H6 (PRIOR ART) 23 I EXPOSURE THROUGH gsx osuks THROUGH F/LTER x 7 6 24 FILTER v v 26 PRINT/N6 TRANS MISS/0N TRANSMISSION XPO.SURE 30 (EXPOSURE .33 success /I/ PRINT/N RA SMISSIO v s E 29 :ExP0su/QE CORRE'CTEO F T r i POSX L 1' Tm/YsM/ssm/Y L 3/ YULE r {EXPOSURE INVENTOR BYI A TTORNE Y Jan. 6, 1942. J. A. c. YULE COLOR comwcnon Filed April 30, 1940 2 Sheets-Sheet 2 206 EXPOSURE Lo's Exbasuks EXPOSURE JOHN/4. C. VULE INVENTOR M22 z m B Y ATTORNEY negatives are made on Patented Jan. 6, 1942 John A. C. Yule, Rochester, Eastman Kodak Company,

N. Y., assignor to L Rochester, N. Y., a

corporation of New Jersey Application April30, 1940, Serial No. 332,564

5 Claims.

This invention relates to color correction in color photography and in photo-mechanical color processes.

Three-color processes f color reproduction and four-color processes which are merely threes color processes modified by the introduction of a separate control for black can be .divided into additive processes and subtractive processes, as is well known. Certain errors which exist only in additive processes or are associated with the additive portion of subtractive processes have been 'described by many authors, for example those listed in the bibliography on page 400, of the Journal of the Optical Society of America, vol. 28, No. 11, November 1938.

It is an object of the present invention to correct these particular errors.

Gertain methods of such correction are already known. It is a particular object of the invention to provide methods which are simple and more flexible and which give correction over a greater range of tones than do previous methods.

Specifically, the errors in additive processes are due to the existence of negative portions on the theoretically correct spectral sensitivity curves for making three-color separations as computed from the tri-stimulus values of a set of primaries. Since it is not practical to have a photographic emulsion, either with or without a filter, with negative response response to others, it is proposed to record the positive and negative portions separately and to subtract them. It has been theoretically shown that this step should mathematically be the linear subtraction of quantities linearly proportional to exposure if the correction is to be exactly that required in additive color processes.

According to the present invention, a color separation positive corresponding to one of the correct spectral sensitivity curves is produced bymaking two color-separation negatives of the original, one corresponding .to the positive portion and the other to the negative portion of that correct spectral sensitivity curve. Both of these photo-sensitive layers whose response (indicated by 'the characteristic curve of the emulsion used) is substantially that in which density is a direct linear function of exposure. A thin positive is then made from this second negative (the one corresponding to the negative portion of the curve) on an ordinary photo-sensitive layer, 1. e. one which, except for the toeand shoulder of its characteristic curve, gives a density which is alinear function of the logarithm of exposure. This thin positive is used to mask the first negative.

In most masking processes, a corrected negative consists of a combination of a negativerand a positive in register. A corrected negative according to the present invention consists of such a to certain colors and positive then ' corresponding point of correct curves.-

combination in which the density of each point of the negative is a positive direct linear function of the reflectivity 'or transmission of.the the original to light whose color corresponds to the positive portion of one of the correct spectral sensitivity curves and the density of each point-of the positive being a negative direct linear transmission of the corresponding point of the original to light whose color corresponds to the negative portion of this particular one'1of the on of a transparency and reflectivity of an opaque colored picture are transmission (T) I or the reflectivity'(R) and density is the logarithm of opacity, that is,

I I log or log A direct linear function refers to a relationship of the type of 11=A=+B, being a positive or negative function according as the constant A is positive or negative. An inverse linear function is of the yp By varying invention may of the exposure methods of introducing color correction is, of

invention and the invention course, one of the main advantages of the present invention. Other objects and advantages of the itself will be fully understood from the following description when read in connection with the accompanying drawings, in which:

Fig. 1 shows a set of theoretically correct spectral sensitivity curves.

Fig. 2 shows one of these curves separate from the others.

l Fig. 3. is a flow chart illustrating one process according to the invention.

Fig. 4 is a flow chart illustrating a prior process for accomplishing the same result.

Figs. 5, 6, and '7 are graphs comparing the response with those required. by prior methods.

function of the reflectivity or the exposure or printing times, the

of ordinary photographic emulsions, 'with those required by the present invention and sensitivity curves attain their maximum and half-maximum values will be within 10 millimicrons of the values indicated by the curves in this figure. In other words, the positive portions' of the theoretical spectral sensitivity curves can vary only very slightly from the values shown in Fig. 1 even though quite a wide choice of reproduction primaries is available. On the other hand, the limits and magnitudes of the negative portions of the curves depend very critically on the choice of reproduction primaries. These negative portions correspond approximately in magnitude and spectral location to the areas and locations of the portions of the well known color mixture diagram lying within the spectrum locus but outside the triangle whose apices represent the reproduction primaries. If one could obtain photographic emulsions with or without filters whose responses correspond to these curves III, II, and I2, the fundamental errors of additive processes would be eliminated. However, it is not practical to have emulsion-filter combinations of this type and the purpose of the present invention is to simulate the same effect.

In Fig. 2 one of the curves (I2) is shown separately. The positive portion X of this curve I2 can be simulated in spectral distribution of response approximately by certain filter-emulsion combinations. This is well known. The negative portion Y as represented by the part I3 of the curve I2 may be simulated by an emulsionfilter combination whose curve I4 gives an area Y which is the mirror image of thearea. Y. A proper combination of these two eiTects subtracting one from the other will result in the efiect corresponding to the curve I2 itself.

However, the usual type of subtraction, namely masking with ordinary emulsions does not result in the subtraction of exposures as required by Fig. 2. That is, the subtraction of densities is actually the division of exposures.

According to the invention as illustrated in Fig. 3, a colored original I5 is photographed as illustrated by the arrows I6 and I! through filters corresponding to the portions X and Y of the curveIZ to form color separation negatives I8 and I9. In the example illustrated in Fig. 2, the region X corresponds substantially to orangered with a small amount of blue and the Y is a bluish-green or at least corresponds to a low' exposure to a bluish-green. However, the invention is equally applicable to any of the curves III, II or I2 shown in Fig. 1, or to any of the slight variations of these curves which are theoretically possible as mentioned above. Even the positive portions of each of the curves are difierent from For example, a filter with a primary colors. transmission corresponding to the X portion of curve I2 would appear yellow-brown. Even if the positive portions are considered to be a kind of primary color, the negative portions are definitely not since Y for example does not overlap at all with X as primary colors must. The portions X and Y are, in a sense, complementary. The photographic materials selected for making the color separation negatives I8 and I 9 are such that their responses give a density which is a linear function of exposure rather than of the logarithm of exposure as in ordinary emulsions.

By printing on an ordinary emulsion, a positive 20 is made from the color separation negative I8. The positive 20 is thin (i. e. not dense) and corresponds to the curve I4 in Fig. 2. This thin positive 20- is then placed in the position 2I in masking register with the color separation I9. The masking combination of the negative I! and the positive 2| forms a novel combination which constitutes a corrected color separation negative whose color corresponds to all of the curve I2. A positive 22 printed from this corrected negative constitutes a corrected color separation positive. The correction is of the type required by additive processes. I

According to a preferred embodiment of the invention the photo-sensitive layer selected for making the corrected positive 22 has a response complementary to that of the negative I9 so that correct tone reproduction is obtained throughout the whole range of tone values. The best method of determining what positive material to select is to compute the required response according to the graphic method described by L. A. Jones in the Journal of the Society of Motion Picture Engineers, vol. XVI, number 5, 1931, page 568, to 599, and elsewhere.

In Fig. 4 a prior process for accomplishing similar results is illustrated in which the original 23 is photographed as indicated by arrows 24 and 25 to form color separation negatives 26 and 21 on photo-sensitive layers whose transmission is proportional to exposure, that is, on layers whose transmission is a negative linear function of exposure. A positive 28 printed from the negative 26 on a similar photo-sensitive layer is used to print an additional exposure on the final corrected positive 33. The photo-sensitive material to form the final positive 33 must also have a response in-which transmission is a linear function of exposure; this positive 33 is made by successive printing illustrated by the arrows 29 and 30 and by the broken line blocks 3| and 32. The simplicity of masking compared to the masking of successive exposures is obvious. There is also another fundamental advantage of the present arrangement compared to that illustrated in Fig. 4 (best described by reference to Figs. 5, 6, and 7), namely, the materials required are more easily and exactly obtainable. That is, it is easier to get an emulsion whose density is a linear function of exposure than to get one whose transmission is a linear function of exposure.

In Fig. 5 density is plotted against the logarithm of exposure in the Well-known way. An

ordinary emulsion has a curve of the form A with a toe 4|. Throughout most of the exposure range, density is a linearfunction of the'logarithm of exposure in anordinary emulsion as illustrated by the fact that this curve A is a straight line. This same curve A is plotted in Fig. 6 inwhicli transmission is plotted against exposure and is also plotted in Fig. 7 in which density is plotted against exposure. The form the curve takes in these figures is best understood-by direct reference to the drawings themselves. According to the prior art illustrated in Fig; 4 a photo-sensitive layer is required in which the transmission is a negative direct linear func.

. function of exposure.

illustrated by the straight line B in Fig. 6. This curve B also appears in Figs. and '7. According to the present invention there is required a photo-sensitive layer whose density is a positive direct linear'function of exposure as illustrated by the straight line C in Fig. 7. Each of these curves A, B, and C is merely representative of thousandsof families of curves. The particular ones selected all pass through a point 40 having the same density for the same exposure, but this is merely to illustrate the difierences in the types of responses.

, Since photographic emulsions having the form of response illustrated by the curve B are not available, it has been proposed to work on the toe of an ordinary emulsion. now made with reference to the curve C, but since the curve C comes more nearly approximating the normal curve A, it is possible to match the curve C with an ordinary emulsion over a greater range than curve B can be matched. Of course, the best approximation to either curve B or curve C will not match these curves exactly at any one point, but the simplest way to compare the ease of matching is to assume that-an ordinary emulsion is selected which exactly matches the curves over the toe region. The shape of a toe of a normal curve is illustrated by the broken line 4| in each of the Figs. 5, 6, and '7. when a normal emulsion is selected which matches the curve B over .the toereglon, this response leave the curve'B at the point 43 and will have the form shown by the broken line 45. A similar situation holds in the case of the curve C, the actual response leaving the curve C at the point 42 and following a broken line 44.

However, the point 42 is further along the curve C than is the point 43 along the curve B and the departure represented by the portion 44 from the curve C is much less than the departure of the broken line 45 from the curve B. This is especially evident from Figures 6 and '7. p

In practice, by selecting a high contrast normal emulsion which has an exceptionally long tion of exposure as toe and which deviates very slightly from an ex act match in the extreme end of the toe (but does not deviate beyond tolerance limits) an emulsion can be selected which practically matches the curve C throughout its whole range. A similar procedure in the case of curve B still leaves much to be desired because of the rapidity with which the normal response curve deviates from the curve B. In other words, it is much easier. to get an emulsion in which density is a linear function of exposure than it is to get an emulsion in which the transmission is a linear This combined with the fact that masking is a much easier form of color correction than the making of subsequent .exposures forms the main advantages of this invention.

According to the referred embodiment of the priesent invention, the correct'response'for the vfinal corrected positive is computed graphically by Jones method and this emulsion is used in making the corrected positive 22. The required shape is roughly that illustrated by thecurve D in Fig. 8. I have found that thiscurve D' can be matched-more exactly by an ordinary emulsion than can curve B although not as easily as can curve C. In order to give linear to'ne reproduction, the contrast of the positive as illustrated by curve D, should be inversely proportional to the contrast A similar proposal is of the corresponding point of the original to light of the negative at least not limited to this example,

over a substantial range of exposures. This is obvious to those who commonly employ Jones graphic method described above.

Having thus described the preferred embodiment of my invention and illustrated a typical example thereof, 1 wish to point out that it is but isof the scope of the appended claims.

Whatv I claim and desire to secure by Letters Patent of the United States is:

1. In a process for the reproduction of a colored original, the method of making a color separation positive which comprises making a color separation negative of the original, the color corresponding approximately to the positive portion of one of the correct spectral sensitivity curves of a three-color additive process, making a second color separation negative of the original the color corresponding approximately to the negative portion of said one of the curves, both of said negatives being. madeon photo-sensitive layers whose responses are substantially those in which density is a positive direct linear function of exposure, making a positive from the second negative on a photo-sensitive layer with ordinary response, and making a positive from the first negative masked by the positive made from the second negative.

2. A method according to claim 1 in which the final positive is made on a photo-sensitive layer whose contrast is inversely proportional over a substantial range to that of the firstmentioned color separation negative whereby the product of the two contrasts gives linear tone reproduction,

3. In masking combination, a primary color separation negative of a colored original, the density at any point of the negative being a positive direct linear function of the intensity of the primary color light from the corresponding point of the original and a color separation positive in register with the negative,,the density at any point of the positive being smalland a negative'direct linear function of the intensity of a color from the original which is substantially complementary to said primary color.

4. In masking combination and in register, a color separation negative and a color separation positiveof a colored original, the density of any point of the negative being a positive direct linear function of the reflectivity or transmission whose color corresponds to the positive portion of one of the correct spectral sensitivity curves of a three-color additive process and the density of any point'of the positive being a negative linear function of of the corresponding point of the original to light whose color corresponds to the negative portion of said one of the curves. i

5. In a process for the reproduction of a colored original, the method of making a color separation positive, which comprises making two color separation negatives of the original corre-' sponding to different layers whose responses are which density is a positive tion of exposure, making a positive-from oneof the negatives on a photo-sensitive layer with ordinary response and making a positive from the other negative masked by the positive made from said one ofthe negatives.

some a. o.

colors on photo-sensitive direct the reflectivity or transmission substantially those in rect linear func 

